1. Field of the Invention
The present invention relates to braking systems for heavy duty vehicles, and in particular to an improved diaphragm-based spring brake actuator which provides significantly increased braking force from a spring brake assembly having a size that is the same as or smaller than existing brake assemblies.
2. Description of the Prior Art
Various forms of pneumatic vehicle spring brake actuators have been introduced over the years primarily for use in the trucking industry. A typical double diaphragm air brake actuator includes two portions: an operator controlled service brake portion which is used for slowing or stopping a vehicle, and an emergency or parking brake portion which automatically engages when air pressure is removed. A typical service brake portion is characterized by a closed housing which contains a movable diaphragm stretched across the inside. One side of the diaphragm is closely associated with a centrally located pressure plate attached to a slidable push rod which extends out of the housing for attachment to the brakes of the vehicle. On the other side of the diaphragm a sealed chamber is formed within the housing.
An opening is provided in the sealed service brake chamber for connection to a pneumatic (air) pressure source usually provided by an on-board air compressor. The brakes of the vehicle can be applied by introducing sufficient pneumatic pressure into the sealed chamber to act against the service brake diaphragm which moves the plate, pushing the push rod out. A small return spring is ordinarily provided inside the service brake housing around the push rod and adjacent to the pressure plate to urge it to retract when the air pressure behind the diaphragm is reduced.
A typical emergency brake portion of an air brake actuator is attached in axial alignment with or made a part of the service brake assembly. The emergency brake is a separate closed housing which contains a heavy main compression spring and a second movable diaphragm creating a second sealed chamber. The emergency brake diaphragm is in contact with a second pressure plate which is also attached to or directly associated with the slidable central push rod of the service brake.
The second sealed chamber is formed inside the emergency brake housing on one side of the diaphragm, and the heavy main compression spring is deployed on the opposite side. As with the service brake, the sealed chamber of the emergency brake is connected to the on-board pneumatic source of the vehicle. As long as sufficient air pressure is provided to the sealed chamber, the diaphragm in the emergency brake will remain fully extended thereby compressing the large spring. However, should pressure fall, or should there be a leak in the sealed chamber, the diaphragm will be unable to hold the large compression spring in place. When this occurs, either slowly or quickly, the large compression spring will move the second pressure plate causing the push rod to be extended out thereby applying the brakes of the vehicle.
Under normal conditions, when the vehicle is parked, the air pressure to the emergency brake portion is cut off causing the large compression spring to apply the brakes.
In the transportation industry, it is becoming ever more desirable to provide more powerful spring brake actuators without changing their size. Increasing load sizes, new regulations and other factors have created a need for additional power in a spring brake with the same dimensional profile as existing double diaphragm spring brakes.
A stronger spring brake which takes up the same or a smaller space can result in great savings in the transportation industry. Under present regulations, a loaded truck must be able to apply its brakes and hold its position on a twenty percent (20%) grade. For many heavy vehicles, in order to accomplish this requires additional brake actuators and/or additional axles with brake actuators on them. With stronger brake actuators, fewer of them are needed to bring or hold such a vehicle at rest, thereby saving the cost of the additional brake actuators and/or additional axles.
There is also a need for a more powerful spring brake which fits into a smaller space. This need is driven by such factors as the installation of vehicle air suspensions, lowered floor heights, shorter wheel bases, and the addition of new and bulky chassis equipment. All of these factors compete for the same space occupied by the spring brake.
A spring brake assembly of smaller size which provides the same power as a larger assembly will also reduce weight and cost. A truck tractor and semi trailer may use 8 spring brake actuators on its axles. Replacing these with smaller units having the same strength that are two pounds lighter will reduce the weight by 16 pounds. While this may not seem significant at first blush, a liquid hauling vehicle is frequently loaded to the exact legal limit. Over the life of that vehicle, the 16 pound reduction will convert to thousands of dollars of hauling revenue.
Stronger brake assemblies deployed in the same space can improve the stopping characteristics of a vehicle thereby potentially increasing the gross vehicle weight allowance for the vehicle (i.e. more payload).
Existing service brake assemblies have been designed for attachment to the brake system of a heavy duty vehicle. The end of the service brake push rod is typically attached to a clevis which is, in turn, attached to the end of a slack adjuster arm located on a cam shaft which makes up part of the foundation brake of the vehicle. The push rod is moved in and out of the service brake assembly using pneumatic pressure as described above in order to operate the brakes of the vehicle. As this occurs, in some situations the push rod and clevis move the end of the slack adjuster through a slightly arcuate path around the cam.
For decades, the pressure plates used in existing diaphragm-based spring brake actuators have been relatively small in comparison to the overall profiles of these units. In a typical brake actuator, the pressure plate in the service brake chamber has approximately the same diameter as the pressure plate in the emergency brake chamber. The edges of such pressure plates have traditionally been restricted to the central portion of the brake chamber, presumably to allow sufficient space around the edges of the plates for the diaphragm to fold over itself. However, these traditional wide tolerances that have developed over time are far more than is necessary for the diaphragm to function properly, and have unnecessarily limited the sizes of the pressure plates used, and therefore unnecessarily inhibit the potential force that can be delivered to the push rod by the spring brake actuator.
The present invention is a departure from traditional diaphragm-based spring brake actuator assemblies which allows for the delivery of more force to the push rod without increasing the size of the actuator unit. One embodiment of the invention allows for the use of a stronger heavy main compression spring in the emergency brake chamber to provide greater emergency or parking brake force to the push rod. This is accomplished through novel changes to the design of the emergency brake chamber which allow it to more efficiently hold off the spring. A stronger emergency spring gives the brake actuator a greater capacity to hold a vehicle in place while parked on a grade. Another embodiment of the invention employs similar novel changes to the design of the service brake chamber which allow it to operate more efficiently when braking pressure is introduced.
In the present invention, the pressure plate deployed inside either the emergency housing or the service brake housing, or both, is significantly larger than the corresponding plate(s) found in existing units having the same dimensional profile. The plate(s) of the present invention have a greater diameter and a larger circumference thereby defining a larger area.
With respect to the emergency spring brake, the size of the pressure plate is directly proportional to the amount of force needed to hold off the large compression spring in the emergency brake housing. According to the formula F=PA, the force (F) exerted against the compression spring is equal to the amount of pressure (P) exerted by the chamber multiplied by the area (A) of the pressure plate over which it is exerted. Thus, increasing the size of the pressure plate increases the area (A) over which the pressure (P) is exerted, thereby increasing the force (F) against the spring. For illustrative purposes and by way of example only, and without limiting the scope of the appended claims herein, a pressure (P) of 60 pounds per square inch (60 psi) exerted against a pressure plate in the emergency brake housing having an area of 30 square inches results in a force of 1,800 pounds. In this example, if the area of the pressure plate is increased to 35 square inches, the resulting force of the spring that may be held off increases to 2,100 pounds. Thus, by simply increasing the surface area of the pressure plate, in this example an emergency brake spring that is over 14% stronger may be used (i.e. held off). Typical increases provided by the present invention are in the range of about twenty percent (20%).
The availability of higher pressure (P) will also increase the amount of force (F) available to hold off the emergency brake spring. Thus, by increasing the surface area (A) of the pressure plate alone or in conjunction with increasing the available pressure (P), a much stronger spring may be used in the emergency brake.
With respect to the service brake, the size of the pressure plate therein is directly proportional to the amount of force applied to the push rod. Again, using the formula F=PA, the force (F) applied to the push rod is equal to the amount of pressure (P) exerted by the chamber multiplied by the area (A) of the pressure plate over which it is exerted. Thus, increasing the size of the pressure plate increases the area (A) over which the pressure (P) is exerted, thereby increasing the force (F) applied to the push rod. For illustrative purposes and by way of example only, and without limiting the scope of the appended claims herein, a pressure (P) of 60 pounds per square inch (60 psi) exerted against a pressure plate in the service brake housing having an area of 30 square inches results in a force of 1,800 pounds applied to the push rod. In this example, if the area of the pressure plate is increased to 35 square inches, the resulting force applied to the push rod increases to 2,100 pounds. Thus, by simply increasing the surface area of the pressure plate, in this example the service brake becomes 14% more efficient (i.e. stronger). Typical increases provided by the present invention are in the range of about twenty percent (20%).
The present invention facilitates increasing the size of the either the emergency brake pressure plate or the service brake pressure plate, or both, by incorporating one or more of the following features. First, the cylindrical walls of the spring brake housing may be made more vertical, more parallel to the orientation of the push rod, and/or more nearly perpendicular to the orientation of the pressure plate inside the housing. Next, the space between the outside circumferential edge of the pressure plate and the inside of the cylindrical wall of the brake housing (this space sometimes hereafter referred to as the xe2x80x9cgapxe2x80x9d) may be reduced to a size that is as small as about two and one half (2xc2xd) times the thickness of the diaphragm material, or even smaller (e.g 2xc2xc times said thickness), thereby providing room for a larger pressure plate. Next, the diaphragm itself may be made of very thin material in order to further minimize the size of the above described gap in order to maximize the size of the pressure plate. Next, axial movement of the main compression spring may be minimized by minimizing side load exerted by said spring. This is accomplished by grinding down a portion of the surfaces of the end spring coils (the coils at the top and at the bottom of the spring) so that these coils seat more predictably against the housing and pressure plate. Finally, configuring the shape of the pressure plate to nest with an adaptor plate located on the central shaft of the brake actuator helps keep the pressure plate in central alignment. A bushing/seal retainer may also be employed in the center of the spring housing to help align the larger pressure plate in order to prevent it from drifting sideways. Each of these features, used alone or in conjunction with each other, allows for deployment of a larger pressure plate which can then be used to hold off a stronger spring in the emergency brake housing, or to provide more force to the push rod in the service brake housing.
The use of more vertical cylindrical walls in the present invention increases the interior cross sectional area of the emergency brake housing, thereby allowing for the surface area of the pressure plate to also be increased. This is accomplished without raising the height or width of the cylinder; thus, the overall profile of the brake actuator remains the same.
Through experimentation, it has been determined that the size of the pressure plate may be increased until the above-described gap between the circumferential edge of the pressure plate and the inside wall of the housing is as small as two and one half (2xc2xd) times the thickness of the diaphragm material without any significant degradation in diaphragm performance. Existing brake actuators unnecessarily provide much larger gaps between the edges of the pressure plate and the walls of the housing which range from four and one half (4xc2xd) up to seven (7) times the thickness of the diaphragm material. In the present invention, the surface area of the pressure plate that is gained by closing this gap is substantial. When combined with more vertical cylindrical walls, even more space is made available for the pressure plate.
The use of thinner diaphragm material allows the edges of the pressure plate to extend even closer to the cylindrical walls of the housing, thereby allowing for an even greater increase in the surface area of the pressure plate. Existing brake actuators use diaphragm materials having an average thickness of 0.125 inches, a tight one having a gap of 0.57 inches between the edge of the pressure plate and the wall of the housing (about 4xc2xd times the thickness of the diaphragm). This gap may be reduced, as above, in the present invention down to as small as 2xc2xd times the diaphragm thickness, or even smaller (e.g. 2xc2xdxc3x970.125=0.3125 inches; 2xc2xcxc3x970.125=0.2813 inches). For illustrative purposes and by way of example only, and without limiting the scope of the appended claims herein, if the thickness of the diaphragm material is reduced to 0.09 inches, then the gap may be further reduced to 2xc2xdxc3x970.09=0.225 inches (or 2xc2xcxc3x970.09=0.2025 inches), providing even more room for a larger pressure plate.
Maintaining proper alignment of the larger pressure plate of the present invention is important. This may be accomplished in one or more of several ways. First, an adaptor plate may be employed on the central shaft of the brake actuator on one side of the diaphragm which works in conjunction with a recessed area on the underside of the pressure plate on the other side of the diaphragm. As the pressure plate moves up and down, this adaptor plate nestles through the diaphragm into the recessed area, keeping the pressure plate in central alignment. Alignment may also be improved through the use of a bushing/seal retainer in the center of the spring housing. Alignment may be further improved by reducing the side load of the main compression spring by grinding down the exterior surfaces of the end coils of the spring. Traditionally, such springs have a side load of 6 to 8 percent; in the present invention, reducing this load to 2 or 3 percent greatly improves alignment of the main spring in the emergency housing.
It is to be noted that the improved performance of the diaphragm-based brake actuators of the present invention is accomplished using the same circumferential dimensions as existing brake actuators using common membrane diaphragm materials. The diaphragm is not attached in the center of the actuator, it does not use a moving wall, and it does not have any opening or hole in the center thereof.
Historically, the effective surface area of spring brake pressure plates has been standardized into different types (9, 12, 16 20, 24, 30 and 36), each type providing an incrementally larger braking strength. This allows for standard components and parts to be manufactured for each type. For each type, there is also an incrementally larger associated profile (size) for the brake actuator. Using the design of the present invention, a smaller type (e.g. 24) having a smaller profile may have the strength of a larger type with a larger profile (e.g. 30). A smaller unit utilizing the features of the present invention may be employed as a replacement for a larger type, but requiring a smaller space. In addition, the present invention now makes a new type 43 unit available in the space occupied by a type 36.
It is therefore a primary object of the present invention to provide a stronger diaphragm-based spring brake actuator unit without increasing the overall size of the unit.
It is also a primary object of the present invention to provide a diaphragm-based spring brake actuator unit that is able to hold of a stronger emergency brake spring without increasing the overall size of the unit.
It is a further primary object of the present invention to provide a diaphragm-based spring brake actuator unit that is able to provide more force to the push rod from the service brake assembly without increasing the overall size of the unit.
It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having at larger pressure plate inside of either the emergency brake housing, the service brake housing, or both.
It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having more vertical cylindrical walls on the brake housing to accommodate a larger pressure plate inside.
It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having a very tight gap between the outside circumferential edge of the pressure plate and the inside of the cylindrical wall of the emergency brake housing to provide more room for a larger pressure plate inside.
It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having a diaphragm made of thinner material in the emergency brake housing to provide more room for a larger pressure plate inside.
It is a further object of the present invention to provide a smaller, stronger diaphragm-based spring brake actuator unit in order to allow for more room for air suspensions and other parts underneath the vehicle to which it is attached.
It is a further object of the present invention to provide a stronger diaphragm-based spring brake actuator unit that is retrofittable onto existing brake assemblies.
Additional objects of the invention will be apparent from the detailed descriptions and the claims herein.